This application claims the priority of Japanese Patent Application Nos. 10-96866 filed on Mar. 24, 1998 and 11-36497/1999 filed on Feb. 15, 1999, which are incorporated herein by reference.
The present invention relates to a sealed alkaline-zinc storage battery comprising a tubular positive electrode, a separator, a negative electrode disposed within the tubular positive electrode with the separator sandwiched therebetween and an alkaline electrolyte, in which the positive electrode has a capacity smaller than a capacity of the negative electrode at least in initial charge-discharge cycles, namely, the battery capacity is controlled by the capacity of the positive electrode. More particularly, it relates to improvement of the negative electrode for the purpose of improving the charge-discharge cycle performance of such a battery.
A conventional sealed alkaline-zinc storage battery uses zinc as the negative electrode active material. Since zinc has a small electrochemical equivalent and has a base potential, an alkaline storage battery with a high energy density can be obtained by using zinc as the negative electrode active material. When zinc is used, dendrite (electrodeposited crystal with a branching treelike appearance) is grown during charge. When the dendrite is grown to penetrate through the separator, an internal short-circuit is caused. Accordingly, in order to avoid this problem, it is necessary to use a separator with a large mechanical strength, such as a laminated separator obtained by laminating plural separators, in a practical battery.
Sealed alkaline-zinc batteries are classified into, for example, a battery using a spiral electrode body obtained by winding a positive electrode and a negative electrode (zinc electrode) together with a separator sandwiched therebetween (hereinafter referred to as the "spiral type battery") and a battery using a cylindrical electrode body obtained by disposing a negative electrode within a cylindrical positive electrode with a separator sandwiched therebetween (hereinafter referred to as the "inside-out type battery").
The spiral type battery is disadvantageous in its high manufacturing cost because the structure of the spiral electrode body is complicated and a large amount of expensive separator such as a laminated separator is necessary. Also, since the spiral type battery uses a large amount of separator, the amount of an active material to be packed is unavoidably decreased. Accordingly, the spiral type battery has another disadvantage that the energy density is largely lowered due to the separator. Moreover, the spiral type battery adopts a system in which oxygen generated from the positive electrode during charge is absorbed by the negative electrode, but the oxygen cannot be smoothly absorbed by the negative electrode because the separator is disposed between the positive electrode and the negative electrode. Accordingly, the spiral type battery has still another disadvantage that the pressure within the battery can is so increased through repeated charge-discharge cycles that leakage can be easily caused.
On the other hand, the inside-out type battery is advantageous in its low cost because the structure of the cylindrical electrode body is simple and there is no need to use a large amount of expensive separator. Also, since there is no need to use a large amount of expensive separator, the energy density of the inside-out type battery is advantageously slightly lowered due to the separator. Moreover, it is possible to prevent oxygen from being generated from the positive electrode during charge by controlling the capacity of the zinc electrode to fall within a range where the charge-discharge reaction of the positive electrode (positive electrode active material) is reversible.
For example, Japanese Patent Publication No. 50-2251/1975 discloses an alkaline-zinc storage battery in which the capacity of the negative electrode is controlled to fall within a range where the charge-discharge reaction of a positive electrode mainly including manganese dioxide (MnO.sub.2) is reversible. When manganese dioxide discharges 0.4 or more electrons per 1 mole, irreversible trimanganese tetraoxide (Mn.sub.3 O.sub.4) is produced, which does not return to manganese dioxide by charge, and thus, the charge-discharge reaction of the positive electrode becomes irreversible. On the basis of this fact, the capacity ratio between the positive electrode and the negative electrode is controlled in this battery so that manganese dioxide cannot discharge 0.4 or more electrons.
Furthermore, Japanese Laid-Open Patent Publication No. 62-143368/1987 discloses an alkaline-zinc storage battery including 5 through 20 parts by weight of silver oxide based on 100 parts by weight of manganese dioxide used as the positive electrode active material. When this battery is charged at a constant voltage with the charge voltage set at a predetermined value or lower or is charged at a constant current with the charge termination voltage set at a predetermined value or lower, the generation of oxygen from the positive electrode during charge can be suppressed.
However, both the conventional alkaline-zinc storage batteries are batteries where the battery capacity is controlled by the capacity of the negative electrode, namely, the capacity of the positive electrode is larger than that of the negative electrode (hereinafter referred to as the "negative electrode control type" batteries). Therefore, these batteries have the problem that the battery capacity is largely lowered through repeated charge-discharge cycles. The battery capacity of a negative electrode control type battery is thus largely lowered for the following reason:
Formula (A) represents a charge-discharge reaction in an alkaline-zinc storage battery using manganese dioxide as the positive electrode active material. Formula (B) represents equilibrium of zinc. In Formulas (A) and (B), a rightward arrow indicates the charge reaction and a leftward arrow indicates the discharge reaction. EQU 2MnOOH+Zn(OH).sub.2.revreaction.2MnO.sub.2 +Zn+2H.sub.2 O (A) EQU Zn(OH).sub.2 +2OH.sup.-.revreaction.Zn(OH).sub.4.sup.2- (B)
As is shown in Formula (A), since water is produced in the charge reaction, the concentration of hydroxide ions is decreased by charge. When the concentration of hydroxide ions is decreased, the equilibrium represented by Formula (B) shifts leftward. Therefore, the solubility of zinc is decreased, and a charge failure can be easily caused, and hence, more hydrogen can be generated from the negative electrode. On the other hand, since water is consumed in the discharge reaction, the negative electrode can be easily passivated through repeated charge-discharge cycles. (Passivation is a phenomenon where discharge becomes difficult because the electrolyte is insufficiently supplied to the reaction site.) The decrease of water results in insufficient supply of the electrolyte to the reaction site. Such charge failure and passivation of the negative electrode in the alkaline-zinc storage battery do not bring a significant problem in the spiral type battery with a small depth of reaction in the negative electrode, but can be a main cause of degradation of the charge-discharge cycle performance in the inside-out type battery with a large depth of reaction in the negative electrode.